Apparatus for continuous measurement and control of flotation conditions

ABSTRACT

A method and apparatus necessary to carry out the method of continuous measurement, recordation, and correction of the varying electrode potential or E.M.F. within an ore slurry containing conditioning components in a mineral flotation system by immersing in cooperative relationship a tungsten oxide electrode and a silver chloride electrode in the slurry, relaying the potential thereby obtained to a recording instrument or to an optimizer or to both and either manually, or automatically by means of the optimizer, adjusting the rate of feed of at least one conditioning component of the slurry to correct the potential or E.M.F. to provide a more efficient process.

United States Patent Hart Dec. 18, 1973 [5 1 APPARATUS FOR CONTINUOUS 3,461,056 8/1969 Maeda et a1. 204/292 x MEASUREMENT AND CONTROL OF 31 2??? 36 Every et all...

- rummon ROTATION CONDITIONS 3,674,672 2/1972 Whitesell 137/93 x [75] Inventor: Porter Hart, Lake Jackson, Tex.

[73] Assignee: The Dow Chemical Company,

Midland, Mich.

[22] Filed: Jan. 20, 1972 [21] Appl. N0.: 219,230

[52] US. Cl. 137/93, 204/1 T, 204/195 R [51] Int. C1,. 60511 21/00 [58] Field of Search 137/93; 204/1 T, 204/195 R, 292, 195 F; 324/29, 30 C [56] References Cited UNITED STATES PATENTS 2,192,123 2/1940 Bennett 204/195 R X 2,822,324 2/1958 Gaylor et a1 204/195 F X 3,104,941 9/1963 Hart et al 137/93 X 3,141,835 7/1964 Rolin et a1. 204/195 R X 3,361,150 l/l968 Homer 137/93 3,410,292 11/1968 Bennett et al. 137/93 From 60//rr7/'// Primary Examiner-William R. Cline Attorney-William M. Yates et a1.

[5 7] ABSTRACT A method and apparatus necessary to carry out the method of continuous measurement, recordation, and correction of the varying electrode potential or E.M.F. within an ore slurry containing conditioning components in a mineral flotation system by immersing in cooperative relationship a tungsten oxide electrode and a silver chloride electrode in the slurry, relaying the potential thereby obtained to a recording instrument or to an optimizer or to both and either manually, or

automatically by means of the optimizer, adjusting the rate of feed of at least one conditioning component of the slurry to correct the potential or E.M.F. to provide a more efiicient process.

3 Claims, 4 Drawing Figures PM. 6 H RM.

O Jf I er T Z8 ADems/ry Tanya/en oxide from rough-er; e/ec fro e -F/uors oar aqueous are .S/urpy 6ono ///oner PATENTEUBEE 18 ms sum NF 3 Qba wibnobbw PAIENIEU utc 18 I975 SHEET 2 BF 3 mx w Q\ ON Om OW ON 8 Q0 OO\ APPARATUS FOR CONTINUOUS MEASUREMENT AND CONTROL OF FLOTATION CONDITIONS BACKGROUND OF THE INVENTION Metal and mineral values are, in many instances, obtained from ore applying flotation principles, by pulverizing the ore, slurrying it with water and known treating agents, blowing air upwardly through the so-prepared slurry while it is pumped through a series of flotation cells designed to recover the desired mineral portion of the ore in a froth, which is formed and carried to the surface by the effect of the air passing upwardly through the treated slurry, and is caused to overflow into a collection system from which it may be either guided through additional floatation cells and thence to designed equipment for thickening, vacuum filtration, and drying operations or may be passed directly to such designed equipment after passing through one series of floatation cells.

One mineral, of increasing economic value that is recovered by flotation techniques, is CaF recovered from fluorspar ore. CaF is used as such as fluxes for metal production and serves as a source of F for its many chemical, pharmaceutical, and industrial uses, e.g. the manufacture of Teflon coatings and Freon refrigerants.

Despite the fact that the recovery of CaF from fluorspar ore by flotation has been long known and practiced, the scientific principles involved and the technology for more efficient operations have lagged. As a result, florspar floatation operations are in need of improved means and processes for measuring and controlling conditioning agents useful in such operations.

The invention helps to meet these needs by providing continuous measurement and automatic control, during the flotation process, of agents employed to aid in CaF recovery, e.g. an agent added to control alkalinity. The agent commonly used for alkalinity control is technical grade Na CO i.e. soda ash, which is usually added as an aqueous solution or slurry.

Present operations are guided by periodically sampling the ore slurry in process, sometimes just after the ore has been ground and admixed with the conditioning agents and sometimes after entrance into flotation cells, or both, by drawing a sample from a mixing tank and/or one or more cells. The samples are then tested on a pH meter and adjustments made as indicated by the pH reading and per cent recovery.

A highly significant condition for good flotation is a controlled pH value, or better described as an optimum varying electrode potential. Known electrodes cannot be used for continuous measurement of varying electrode potential (which is closely related to pH and will he sometimes so designated herein) of ore slurries in the process itself. For example, glass electrodes are broken or at lest scratched and eroded by the slurry so that even with frequent calibration, the results are inaccurate and erratic. Other combinations of active and reference electrodes to measure the electrode potential in an agitated mineral ore slurry have not proved fully satisfactory. Furthermore, it appears that the true electrode potential which forms the most reliable guide to flotation process control (as intimated above) is not strictly a pH value. Other factors affecting potential must be measured. Accordingly herein, the value obtained is more correctly referred to as electrode potential which is actually an E.M.F. inherent in the slurry.

A need accordingly exists for an improved reliable way of quick recognition of an unfavorable condition due to an undesirable electrode potential in the slurry and the correction thereof, preferably an automatic prompt correction, e.g. a change in a feed component, to cause the recovery of desired mineral to tend toward the optimum.

SUMMARY OF THE INVENTION The invention is a method of continuously measuring, regulating, and adjusting the varying electrode potential or E.M.F. of an aqueous mineral slurry in a flotation system of which the preferred operating electrode potential to result in highest recovery of CaF of acceptable quality is previously known, which comprises: (l) inserting a tungsten oxide (or tungsten which becomes oxidized in situ) active electrode and a silver chloride (or silver which on contact with chloride ions forms AgCl) reference electrode, in cooperative relationship, i.e. whereby an E.M.F., established in the slurry between the electrodes, is measured and is converted to microvolts or to other desirable units, which are registered and preferably are recorded on the face of a meter, which may be called a potentiometer and, (2) in accordance with whether the so-obtained potential is less or greater than the known preferred potential, either manually or by means of an electronic signal or pneumatic impulse, activating at least one control valve responsive thereto which thereupon either decreases or increases the amount of the varying electrode potential-modifying agent, thereby returning to and tending to maintain the electrode potential in the slurry at a value within the desired relatively narrow limits.

The electrode assembly of the invention provides high impedence, and accordingly is a good means for measurement of E.M.F. where amperage is at a desirable minimum, thereby resulting also in substantial elimination of polarization and more accurate potential measurement.

DESCRIPTION OF THE INVENTION I have discovered that an electrode potential value is obtained by employing a bare tungsten oxide and bare silver chloride electrode (as opposed to employing a salt bridge) in combination in a slurry containing chloride ions which is more reliable than that obtained by employing any known electrodes in combination. In practice a tungsten rod or bar and a silver rod or bar may be initially employed since the tungsten readily acquires an oxide coating and the surface of the silver in contact with the slurry is readily converted to silver chloride. Reference to the results obtained herein is as a potential difference because, as aforesaid, there is an added but not fully understood factor other than the hydrogen ion concentration (which when measured and calculated as the logarithym of the reciprocal thereof is well known as the pH value) that is of significance.

Tungsten oxide electrode, although likely composed of differing valence levels, will be indicated usually herein for simplicity as W0 (which is the predominant oxide present). W0, electrodes are known for use to obtain pH values under certain conditions. (AgCl reference electrodes have been known for use in association with a salt bridge, that is in a submerged compartment containing an aqueous chloride solution and having a pinhole therein for ionic exchange with the liquid being tested.

Heretofore it has been unknown in ore flotation processes to employ WO in combination with a naked or bare AgCl reference electrode to measure the potential in a fluorspar or other ore slurry. It has also been unknown heretofore to monitor continuously the potential obtained and automatically to adjust that potential to a known optimum value.

One embodiment of the invention is to insert a W and a AgCl electrode (or W and Ag pieces which will convert on the surface of WO and AgCl respectively) in cooperative relationship in a mixing tank into which constituents of the ore slurry, to be subjected to flotation, are intermixed; continuously record the potential in any convenient units including microvolts or such converted to pH (although not strictly such), for general reference; and, by means of pneumatic impulse or by electrical signal be conveyed through a conduit or electric line, respectively, to cause motor valves regulating an alkalinity control agent, e.g., soda ash, inflow to open or shut in accordance with either a desired increase or decrease in potential across the gap between the electrodes.

A second embodiment of the invention is to locate the WO -AgCl electrode pair in one or more convenient locations in continuous contact with the ore slurry in processes whereby there is a continuous registration and recordation in the E.M.F. potential in the slurry and then make manual adjustments of conditions as indicated according to independently ascertained tests that show best correlation between potentiometric values and highest yieldv of quality product.

A further embodiment of the invention is to locate the WO -AgCl electrode pair of stated above-but additionally to relay the values to an optimizer which continuously seeks the known optimum electrode potential to produce the peak, i.e. more efficient process, and

which optimizer automatically changes a component being added to the slurry in the direction of optimization, i.e. to that which gives the best result. Preferably, this embodiment includes automatic adjustment of a process condition that tends to result in the most efficient process. For example, in fluorspar flotation, the optimizer by pneumatically or electrically operated valves responsive to optimization, increases or decreases the proportion of a feed ingredient, e.g. soda ash, in accordance with pneumatic impulses or signals sent out by the optimizer pursuant to its peak-seeking principle.

Although the use of an optimizer as described herein is new and unobvious, suitable optimizers adaptable for use in the preferred embodiment of the invention are available. A particularly suitable one is that available as Model 571 Syncro Optimizeras described and well illustrated in Industrial Controls Operating Instructions for Model 571 Syncro Optimizer MS27071, procurable from Moore Products (30., Spring House, Pa., U. S. A.

THE DRAWING:

The drawing schematically illustrates an embodiment of the invention and shows graphs of improved operation according to the invention as opposed to conventional operation.

FIG. 1 depicts the various parts of the assembled apparatus and working relationship of a flotation system for carrying out the process of the invention of fluorspar ore.

FIG. 2 is a graph whereon the potentiometric or varying electrode values of the slurry in the conditioner, are the abscissa and the per cent recovery of CaF product from that available in the ore, corresponding to the potentiometric values, is the ordinate.

FIG. 3 is a graph similar to FIG. 2 but wherein, for purposes of emphasis of the significance of no optimum control of the ionic potential values, the potential was allowed to increase beyond the value for optimum recovery. The effect of no control is to approach zero recovery on each side of the optimum.

FIG. 4 is a schematic representation of the potentiometric instrument and meter comprising the potentiometer of the invention. Thereon, the hours from 1 to 24 are shown about the outside circumference. Varying electrode potentials are shown by the concentric circles, the potential increasing for each outer larger circle. The meter shows two values: a continuous record, preferably in ink is provided for the actual instant reading and a pointer or guide showing the desirable or optimum potential, either set manually or preferably set by the optimizer as later explained.

In FIG. 1, the numbered parts or items of the system therein illustrated for fluorspar flotation are as follows:

Item 2 is a conveyor belt or apron for crushed fluorspar ore. Items 4, 6, and 8 are supply systems for water, aqueous quebracho solution, and aqueous soda ash, respectively. Line 10 takes water (including any recycled water) with the quebracho solution to the outlet of ball mill l2 and line 11 takes the soda ash solution to the entrance end of ball mill 12 where, together with the ore received off 2, an aqueous slurry comprising fine particle size ore is produced. By means of line 13 the slurry from 12 is led to cyclone separator 14 from which those particles finer than about 200 mesh are passed on through line 16 for continued processing and those coarser than 200 mesh are returned through line 15 to the entrance to ball mill 12 to be reground therein.

The slurry in line 16 passes through meter 19, which by means of electric line 20 records on instrument 21 the total volume of ore being treated per unit of time, and enpties into mixing tank 22, commonly called a conditioner. The slurry is stirred and usually moderately heated in 22.

Electrode T (for tungsten oxide) and electrode S (for silver chloride) comprises the slurry E.M.F., i.e. varying electrode potential, sensing instrument 24 of the invention. Electric line 25 and 25a relay the E.M.F. being produced across the electrode gap of 24 to potentiometer 26 which continuously records the actual or currently existing E.M.F. or electrode potential between the electrodes in whatever units are desirable, e.g. microvolts.

If preferred, line 27 may be used (instead of or coni currently with line 25a) which carries, when desirable,

the E.M.F. electrode potential to optimizer 28 which by computer mechanism monitors continually, through connecting conduit 29 to a second pen or potentiometer 26, the peak or optimum electrode potential for best operation. If the actual electrode potential existing is below the optimum, then by way of conduit 32, control valves 33 and 34 for control of the modifying additive for proper adjustment, e.g. soda ash from reservoir 35, will be opened to increase the potential. 0n the other hand, if the existing potential in the slurry is too high, the optimizer will automatically seek the lower potential by cutting back the input of soda ash.

The slurry from conditioner 22 moves therefrom through line 38 to cell F, the first of a series of rougher flotation cells, referred to collectively as 40. The slurry is then subjected to air entering the bottom of each cell to effectuate an overflow froth of high CaF content which is collected by troughs 41 secured along the lip or upper edge. That which does not overflow, known as tailings, passes on to and out from cell L of 40 through line 42 to the tailings pond.

Density of the slurry in process is ascertained. This can be done conventionally by periodically taking liter samples and weighing them.

However, if desired, the invention described and claimed in patent application, entitled Device and Method of Density Measurement and Control of Liquid Systems of Porter Hart, Ser. No. 219,22], filed concurrently herewith, may be employed to measure and record densities and the density differential at various stages of flotation as desired. In accordance with Ser. No. 219,221, Device and Method of Density Measurement and Control of Liquid Systems (Porter Hart), density sensing probes and connected pressure assemblies convert density values to density differential and that to pressure differential which can be used to show efficiency of the CaF recovery and can be correlated with the optimum electrode potential for best performance.

That slurry which is frothed off from cells 40 known as concentrate, aided usually by minimum additional water sufficient for good flowability, is fed into overflow troughs 41 and moved through line 53 to a thickening settling tank, designated thickener 54. Here it is caused to remain and be subjected to slow agitation during which separation of some water from solids occurs by the water overflowing (which optionally may be reused to slurry fresh ore) while the heavier portion of the wet CaF concentrate settles. This wet CaF is pumped from the bottom of 54 through line 56 to vacuum dewatering-filter system 68 comprising envelopes evacuated internally which, by being secured to a rotating wheel, revolve first approximately 180 through the upper surface of a reservoir of CaF, concentrate, thereby causing wet CaF to adhere thereto and be subjected to vacuum within the envelopes which removes appreciable water therefrom by means of the internal vacuum system, and then to complete the remaining approximate 180 to the 360 revolution above the res ervoir surface in forced contact against scraper blades (while continuing to suck water therefrom), such contact removing adhering concentrate from the exterior of the envelopes. The wet solids so removed fall as wet crumbles on conveyor 60 which leads to a series of dryers 62 which removes substantially all remaining moisture to produce a substantially anhydrous CaF marketable end product of at least about 97.4 percent purity.

Air for the pneumatic system is provided by source 66 through conduit 67 from which branch conduits 68, 69 and 70 leading to instruments 48, 26 and 21, respectively. Conduit 74, from instrument 48, leads to and controls valves 76 and 78, which regulate the water (and thereby the ultimate slurry density) into both the outlet of ball mill l2 and the inlet to the ball mill, respectively.

FIG. 2 shows the electrode potential values obtained according to the invention from a start-up value of zero to the optimum value which is that which results in highest CaF recovery and shows thereafter the desirable manner by which the invention maintains optimum conditions thereafter by the cluster of electrode potential values at highest CaF recovery.

FIG. 3, for contrast, shows that unless the electrode potential values are controlled within a narrow range (as is accomplished by the practice of the invention), but rather are allowed to continue to increase beyond the optimum value of about 10, the CUP-2 recovery drops to about zero.

It should be borne in mind that the optimum electrode values shown on the graphs for highest production varies for all types of ore and also for fluorspar ore, even when mined from the same location. This optimum value must be ascertained for each ore to be processed.

FIG. 4, depicts the essential features of the invention without the necessary accompanying or supplemental parts for practical operation.

EXAMPLES The performance of the improved process and apparatus of the invention in contrast to conventional practice is well demonstrated by the following examples wherein:

Examples 0-1 to C-5 were conducted for comparative purposes according to conventional practice and Example I was conducted in accordance with the invention.

The duration of time for each conventional example and the example of the invention was exactly the same. The conditions applicable to both the conventional practice and that of the invention comprising the average rates of ore feed and of the conditioning or treating agents, the ore analysis, total solids, temperature, the pH value manually taken of the slurry in the conditioner (i.e. just prior to flotation), the automatically recorded varying electrode potential obtained according to the invention and the per cent CaF recovered, based on that originally in the ore, are set out in the table.

The procedure followed for all examples consisted essentially of employing a mill lay-out as shown schematically in FIG. 1, except that in the conventional examples, neither the improved apparatus and method for ascertaining and maintaining the optimum electrode potential nor the optimization in accordance with the invention was employed in the slurry.

In general, fluorspar ore was fed to the mill via conveyor 2 into ball mill l2. Quebracho and soda ash 10 percent aqueous solutions, together with sufiicient water to result in the desired total solids slurry, where added to the ore, a portion of water prior to its entrance into the ball mill for good grinding and a second portion as it left the ball mill to provide the desired solids for flotation. The ore was ground to a particle size predominantly between 200 and 400 mesh in the ball mill. It passed out of the ball mill through line 13 to cyclone separator 14 which diverted particles coarser than 200 mesh through line 15 for re-entrance into the ball mill and the smaller sizes via line 16 (through total ore volume meter 19 which registered on meter 21) to conditioner 22. Meter 19 is calibrated to record only ore solids volume exclusive of the water.

In 22, oleic acid as a collection agent, and as needed, additional soda ash solution were admixed with the slurry.

From the conditioner, the ore slurry was passed to the first, i.e. cell F,of the series of rougher flotation cells and thence from cell to cell of the series designated No. 40 in FIG. 1, (each of which is substantially the same design), while the slurry is subjected to an upward flow of air and continuous agitation to produce a froth whereby the CaF is caused to adhere to the rising foam and froth and be carried as overflow into a collecting trough system positioned to-collect the froth overflow along the edges of the cells and convey it to thickener No. 54 of FIG. 1 from whence the thickened slurry is further processed by having additional water removed by vacuum designated without detail by No. 58 in FIG. 1. The remaining water is removed by heat drying in dryers No. 62.

In the conventional examples, pH values were obtained at least every hour by taking a slurry sample and ascertaining its value manually on a conventional glass electrode pH meter. Changes were thereafter made in the soda ash flow rate as indicated. Accuracy was poor and the delay between an indicated change and the making of such change was far longer than is necessary for good operation.

In the example of the invention, the potentiometric instrument of the invention,which appears to measure something other than pH (which is classically the value of the log of the reciprocal of the hydrogen ion concentration),designated No. 24 in FIG. 1, was employed. The W (T) and AgCl (S) electrodes were immersed secured in the slurry in conditioner 22. Electric lines No. 25 led to a pen on meter 26 which continuously recorded the electrode E.M.F. potential of the slurry. When the reading indicated a needed increase or decrease in an agent, e.g. soda ash, such change could have then been manually made.

However, the preferred practice of the invention and that employed in the example was automatic control. This was accomplished by providing a pneumatic conduit which led to soda ash valves No. 33 and No. 34 which responded automatically to a predetermined optimum electrode E.M.F. For example, when 9.0 had been predetermined to result in best operation for processing a specific ore, then any potentiometric value of less than 9.0 caused a measured increase in soda ash rate to re-establish optimum value. 0n the other hand, a value above 9.0 was too high and therefore the opti- Reference to the examples clearly shows the more consistent higher recovery of high quality CaF product obtained in Example I of the invention than that obtained in comparative Example C-l to C5 The improvement of the invention is even more striking in the stability of the operation as shown by the narrow soda ash limits, the reduced demand for oleic acid, and the stability of the E.M.F. or potential of the slurry in Example I as opposed to the erratic soda ash requirements and wide range of pH values obtained by conventional control. Although not apparent from the table, the practice of the invention results in a definite reduction in the number of man hours required per metric ton of ore processed.

Variations in the practice of the invention from that demonstrated above are possible without going outside of the invention. The improved potentiometric measuring instrument and method of use in flotation systems characterizes the invention. Accordingly, any modification of the flotation system which uses the tungsten oxide-silver chloride electrode immersed in cooperative relationship in a slurry to be or being subjected to flotation techniques and such potential continuously .recorded for purposes of either manual or automatic modification of a processing-modifying ingredient falls within the invention.

For example, the electrode assembly recording system of the invention may be coordinated with a density-pressure cell the values of which are also passed into an optimizer (as described in copending Application Ser. No. 219,221, entitled Device and Method of Density Measurement and Control of Liquid Systems, of Potter Hart, filed concurrently herewith).

If desired, the concentrate which is frothed off the roughers 42 may be subjected to further frothing in additional units substantially of the nature of 42, usually called cleaners, before being passed to the thickenerdewatering filter system and subsequently to the dryers.

Having described my invention, what I CLAIM and desire to protect by Letters Patent is:

1. An apparatus for the continuous and automatic measurement, registration, and correction of the varying electrode potential established in an aqueous ore slurry composition, containing chloride ions and conditioning agents, at a stage prior to its being subjected to flotation, comprising:

an electrode assembly consisting essentially of a tungsten oxide active electrode and a silver chloride reference electrode immersed directly in said mlZ Caused a measurtid decrease Soda ash slurry composition in cooperative relationship to Reference to the table shows the conditions and reprovide an electrode potential between said elecsults of the examples according to conventional practrodes; tice and that according to the invention. indicator means upon which said potential is continu- TABLE Conventional examples E.M.F. or Percent Percent potential in CaFz total ore slurry recovered 10% aq. 10% aq; solids or measured based on Percent Ore, Percent quebra- Oleic soda ash slurry pH values by W02- CaFz CflFz kgm QaF cho sol. acid, solution, in condi- Temp taken AgCl eleecontent in end min 111 ore mL/min. ml./m1n ml./m1n. tioner manually trodes of ore product 45. 3 500 3 1, 200-3, 300 32 120 9. 2-9. 7 Not in use. 53. 3 J7. 4 (i6. 5 500 3-5 1, 600-4, 300 32-33 0. 24). 8 .(10 63. 8 97. 5 64. 1 500 3-5 1, 2004, 000 32 1. 1-9. 8 do 74. 1 97. 4 62. 3 500 3-5 1, 2004, 000 32 120 0. 1-9. 8 H. ,do 63. 6 J7. 4 63. .1 500 3-5 1, 200-3, 800 32-33 120 0. 2-9. 8 .(10 61. 7 97. 3

Example of the invention I 150. 0 76. 1 500 0. 3 1, 400-1, 600 32 110 and 120 9. 2-9. 8 8. 8-9. 2 77. 1 97. 4

*The major proportion of the balance of the ore was CaCOa and the minor proportion was SiOz, the total of both being inversely proportional to the percentages of CaFz present.

ously registered by means of electrical connections from said electrodes; control means for changing the rate of feed of an electrode potential modifying ingredient to said slurry composition in response to said indicator means to maintain a predetermined optimum potential so as to provide maximum mineral recovery;

and means forconveying a signal from said indicator means to said control means.

trode modifying ingredient is automatically adjusted.

UNITED STATES PATENT OFFICE Patent No.

Inventor-(s) Dated December 18, 1973 Porter Hart It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. line Col. line Col. line Col. line Col. line 38, "more" should be changed to -most-.

1, "of" 2nd occ. fihld read on 49 "comprises" should be -comprise 42, "68" should be changed to 58'-.

50, "to" should be changed to --of.

Signed and sealed this 22nd day of October 1974. I

(SEAL) Attest:

McCOY M. GIBSON JR. Attesting Officer 0. MARSHALL DANN Commissioner of Patents 

1. An apparatus for the continuous and automatic measurement, registration, and correction of the varying electrode potential established in an aqueous ore slurry composition, containing chloride ions and conditioning agents, at a stage prior to its being subjected to flotation, comprising: an electrode assembly consisting essentially of a tungsten oxide active electrode and a silver chloride reference electrode immersed directly in said slurry composition in cooperative relationship to provide an electrode potential between said electrodes; indicator means upon which said potential is continuously registered by means of electrical connections from said electrodes; control means for changing the rate of feed of an electrode potential modifying ingredieNt to said slurry composition in response to said indicator means to maintain a predetermined optimum potential so as to provide maximum mineral recovery; and means for conveying a signal from said indicator means to said control means.
 2. The apparatus of claim 1 wherein the control means includes at least one valve for changing the rate of feed of the electrode potential modifying ingredient.
 3. The apparatus of claim 1 wherein a peak-seeking optimizer is included between said electrode assembly and said indicator means and is electrically connected therebetween, whereby the rate of feed of said electrode modifying ingredient is automatically adjusted. 